The present invention relates to a method for pickling of steel in an acidic aqueous pickling solution containing Fe.sup.3+ and Fe.sup.2+. The pickling capability of the bath is maintained by continuous supply of hydrogen peroxide.
In manufacturing of steel, particularly stainless steel, an oxide layer forms at the surface during the annealing, and this layer must be removed. This is normally done by pickling which means that the steel is treated in an acidic oxidizing pickling bath to effect some dissolution of metal under the oxide layer which then comes loose. For a ling time, pickling of stainless steel has often been performed in pickling baths based on nitric acid as an oxidizing agent which, however, has involved emissions of nitrous fumes and nitrates that are detrimental to the environment.
U.S. Pat. No. 4,938,838 discloses addition of hydrogen peroxide for oxidizing nitrite to nitrate in nitric acid based pickling baths. The emissions of nitrous fumes are significantly reduced but are not totally eliminated, and the emissions of nitrates are not reduced at all.
Pickling without nitric acid is disclosed in the U.S. Pat. Nos. 5,154,774 and 5,354,383 and in GB-A-2000196. These processes are based on the fact that Fe.sup.3+ in the pickling bath acts as an oxidizing agent and is reduced to Fe.sup.2+ at the same time as metallic iron in the steel is oxidized to Fe.sup.2+. In order to maintain the oxidation potential in the pickling bath hydrogen peroxide is added to reoxidize Fe.sup.2+ to Fe.sup.3+. A disadvantage of these processes is that the cost for hydrogen peroxide is rather high since a great deal of it does not just react with Fe.sup.2+ but also with other metal ions in the pickling bath, such as Fe.sup.3+, and is then consumed to no use. It is also hard to achieve a sufficiently high pickling rate.